Volcanic Islands [6]
which is now fully recognised as having been a potent factor in the production of the metamorphic rock, and has been called by more recent writers "dynamo-metamorphism."
In this, and in many similar discussions, in which exact mineralogical knowledge was required, it is remarkable how successful Darwin was in making out the true facts with regard to the rocks he studied by the simple aid of a penknife and pocket-lens, supplemented by a few chemical tests and the constant use of the blowpipe. Since his day, the method of study of rocks by thin sections under the microscope has been devised, and has become a most efficient aid in all petrographical inquiries. During the voyage of H.M.S. "Challenger," many of the islands studied by Darwin have been revisited and their rocks collected. The results of their study by one of the greatest masters of the science of micropetrography--Professor Renard of Brussels--have been recently published in one of the volumes of "Reports on the 'Challenger' Expedition." While much that is new and valuable has been contributed to geological science by these more recent investigations, and many changes have been made in nomenclature and other points of detail, it is interesting to find that all the chief facts described by Darwin and his friend Professor Miller have stood the test of time and further study, and remain as a monument of the acumen and accuracy in minute observation of these pioneers in geological research.
JOHN W. JUDD.
CHAPTER I.--ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.
Rocks of the lowest series. A calcareous sedimentary deposit, with recent shells, altered by the contact of superincumbent lava, its horizontality and extent. Subsequent volcanic eruptions, associated with calcareous matter in an earthy and fibrous form, and often enclosed within the separate cells of the scoriae. Ancient and obliterated orifices of eruption of small size. Difficulty of tracing over a bare plain recent streams of lava. Inland hills of more ancient volcanic rock. Decomposed olivine in large masses. Feldspathic rocks beneath the upper crystalline basaltic strata. Uniform structure and form of the more ancient volcanic hills. Form of the valleys near the coast. Conglomerate now forming on the sea beach.
(FIGURE 1: MAP 1: PART OF ST. JAGO, ONE OF THE CAPE DE VERDE ISLANDS.)
The island of St. Jago extends in a N.N.W. and S.S.E. direction, thirty miles in length by about twelve in breadth. My observations, made during two visits, were confined to the southern portion within the distance of a few leagues from Porto Praya. The country, viewed from the sea, presents a varied outline: smooth conical hills of a reddish colour (like Red Hill in Figure 1 (Map 1). (The outline of the coast, the position of the villages, streamlets, and of most of the hills in this woodcut, are copied from the chart made on board H.M.S. "Leven." The square-topped hills (A, B, C, etc.) are put in merely by eye, to illustrate my description.)), and others less regular, flat-topped, and of a blackish colour (like A, B, C,) rise from successive, step-formed plains of lava. At a distance, a chain of mountains, many thousand feet in height, traverses the interior of the island. There is no active volcano in St. Jago, and only one in the group, namely at Fogo. The island since being inhabited has not suffered from destructive earthquakes.
The lowest rocks exposed on the coast near Porto Praya, are highly crystalline and compact; they appear to be of ancient, submarine, volcanic origin; they are unconformably covered by a thin, irregular, calcareous deposit, abounding with shells of a late tertiary period; and this again is capped by a wide sheet of basaltic lava, which has flowed in successive streams from the interior of the island, between the square-topped hills marked A, B, C, etc. Still more recent streams of lava have been erupted from the scattered cones, such as Red and Signal Post Hills. The upper strata of the square-topped hills are intimately related in mineralogical composition, and in
In this, and in many similar discussions, in which exact mineralogical knowledge was required, it is remarkable how successful Darwin was in making out the true facts with regard to the rocks he studied by the simple aid of a penknife and pocket-lens, supplemented by a few chemical tests and the constant use of the blowpipe. Since his day, the method of study of rocks by thin sections under the microscope has been devised, and has become a most efficient aid in all petrographical inquiries. During the voyage of H.M.S. "Challenger," many of the islands studied by Darwin have been revisited and their rocks collected. The results of their study by one of the greatest masters of the science of micropetrography--Professor Renard of Brussels--have been recently published in one of the volumes of "Reports on the 'Challenger' Expedition." While much that is new and valuable has been contributed to geological science by these more recent investigations, and many changes have been made in nomenclature and other points of detail, it is interesting to find that all the chief facts described by Darwin and his friend Professor Miller have stood the test of time and further study, and remain as a monument of the acumen and accuracy in minute observation of these pioneers in geological research.
JOHN W. JUDD.
CHAPTER I.--ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.
Rocks of the lowest series. A calcareous sedimentary deposit, with recent shells, altered by the contact of superincumbent lava, its horizontality and extent. Subsequent volcanic eruptions, associated with calcareous matter in an earthy and fibrous form, and often enclosed within the separate cells of the scoriae. Ancient and obliterated orifices of eruption of small size. Difficulty of tracing over a bare plain recent streams of lava. Inland hills of more ancient volcanic rock. Decomposed olivine in large masses. Feldspathic rocks beneath the upper crystalline basaltic strata. Uniform structure and form of the more ancient volcanic hills. Form of the valleys near the coast. Conglomerate now forming on the sea beach.
(FIGURE 1: MAP 1: PART OF ST. JAGO, ONE OF THE CAPE DE VERDE ISLANDS.)
The island of St. Jago extends in a N.N.W. and S.S.E. direction, thirty miles in length by about twelve in breadth. My observations, made during two visits, were confined to the southern portion within the distance of a few leagues from Porto Praya. The country, viewed from the sea, presents a varied outline: smooth conical hills of a reddish colour (like Red Hill in Figure 1 (Map 1). (The outline of the coast, the position of the villages, streamlets, and of most of the hills in this woodcut, are copied from the chart made on board H.M.S. "Leven." The square-topped hills (A, B, C, etc.) are put in merely by eye, to illustrate my description.)), and others less regular, flat-topped, and of a blackish colour (like A, B, C,) rise from successive, step-formed plains of lava. At a distance, a chain of mountains, many thousand feet in height, traverses the interior of the island. There is no active volcano in St. Jago, and only one in the group, namely at Fogo. The island since being inhabited has not suffered from destructive earthquakes.
The lowest rocks exposed on the coast near Porto Praya, are highly crystalline and compact; they appear to be of ancient, submarine, volcanic origin; they are unconformably covered by a thin, irregular, calcareous deposit, abounding with shells of a late tertiary period; and this again is capped by a wide sheet of basaltic lava, which has flowed in successive streams from the interior of the island, between the square-topped hills marked A, B, C, etc. Still more recent streams of lava have been erupted from the scattered cones, such as Red and Signal Post Hills. The upper strata of the square-topped hills are intimately related in mineralogical composition, and in