356 On a Ternary Geological Classification. [Jiily> 



It can scarcely be considered an argument against the organic 

 origin of marine limestones, that they frequently exhibit no trace 

 of organic structure. Limestones undergo metamorphism sometimes 

 during their very formation in the open sea; how much more so 

 after having been subjected to the action of heat, pressure, perco- 

 lation of water, and other agencies, known and unknown, acting 

 within the crust of the earth. On this subject the testimony of 

 Mr. J. B. Jukes may be considered conclusive. Speaking of the 

 rapid change which coral reefs undergo as observed by him during 

 his voyage in H.M. ship * Fly,' he says, " The surface of a reef 

 when exposed at low water is composed of solid-looking stone, which 

 is often capable of being split up and lifted into slabs bearing no 

 small resemblance to some of our oldest limestones. These slabs, 

 when split up, are frequently found to have a semicrystalline 

 structure, by which the forms and the organic structure of the 

 corals and shells are more or less obliterated." 



As to the origin of the lime in the sea-water, that is a question 

 immaterial to my present purpose, which is to show the essential 

 distinction between the gi'eat group of calcareous formations in all 

 geologic ages, and the strictly mechanical strata with which they 

 are associated.* But before passing to the discussion regarding 

 the relations of this latter class of strata, let us notice the several 

 varieties of marine animals which have chiefly contributed to the 

 formation of limestones, and which we may call " the limestone 

 builders." 



Limestone Builders. — At first sight it might be supposed that 

 nearly all invertebrate marine animals having stony skeletons, or 

 shells, contributed proportionally to the formation of limestones. 

 But when we come to examine the classes of animals which in our 

 own day have contributed to the formation of the calcareous ooze of 

 the Atlantic, or the limestone reefs of the Pacific; and extending 

 our researches back into geologic times, examine the structure of 

 our great limestone masses, we soon perceive that the chief hmestone 

 builders have been animals of comparatively low organization, and 

 embrace but a small portion of the sub-kingdom of invertebraia. 

 They consist for the most part of the calcareous shells of Forami- 

 nifera, associated with the siliceous shields of Polycystina, forming 

 not only the present calcareous ooze of the Atlantic bed but also a 

 large proportion of the Secondary and Tertiary limestones. Next 

 come the polyps or corals {Actinozoa or Anthozoa), ranging down- 

 wards from Palaeozoic times to the present day, and entering largely 

 into the composition of Silurian, Carboniferous, and Jurassic lime- 

 stones. Then the Bryozoa or Polyzoa, largely distributed in the 



* The reader will find the question as to the origin of limn in tlie sea-water, 

 and the apparent increase of calcareous rocks in more recent times, discussed in 

 Lyell s Trinciples of Geology,' 10th edit., p. G08. 



