74 INTRODUCTION. 



deposits belonging to a given period. We may have the compara- 

 tively deep-water deposits of the period only, or we may know 

 nothing but its littoral accumulations. In either case it is clear 

 that there is an imperfection of the palseontological record ; for 

 we cannot have even a moderately complete record of the marine 

 animals alone of a particular period, unless we have access to a 

 complete series of the deposits laid down in the seas of that period. 

 A still more serious imperfection of the record arises where, as 

 commonly happens, the marine deposits of a given period are alone 

 known, and we are left without any knowledge of the lacustrine, 

 fluviatile, and terrestrial deposits of the same period. 



According to the views of Dr John Murray and the Abbe Renard, 

 a very important deficiency exists in the series of sedimentary de- 

 posits known to us as forming the existing dry land, in so far as the 

 series is without any representatives of the peculiar deposits which 

 are now in process of formation in the deep sea. It has been 

 shown by these investigators that between depths of six or seven 

 hundred fathoms down to the greatest depths known, at distances 

 of two hundred miles or more from land, there are now being 

 formed certain remarkable deposits which may be spoken of as 

 " deep-sea muds " and " abyssal clays." The deep-sea " muds " and 

 " oozes " are exceedingly fine, mud-like deposits, which differ from 

 true muds in not being made up of water-worn particles of clay or 

 other mineral substances, and in being largely composed of the skele- 

 tons of minute animal or vegetable organisms. Some of these deep- 

 sea muds are largely composed of the shells of Foraminifera (" Glo- 

 bigerina ooze ") ; others are essentially made up of the siliceous tests 

 of Polycystina and allied organisms (" Radiolarian ooze"); others 

 are built up principally of the shells of Pteropoda (" Pteropod ooze ") ; 

 while others are the result of the accumulation of the flinty envelopes 

 of Diatoms (" Diatom ooze "). The " abyssal clays," again, are red, 

 purple, chocolate-coloured, or brown clays, composed of nearly im- 

 palpable particles, and almost destitute of calcareous matter, but 

 sometimes containing particles of metallic iron or concretions of 

 manganese. These abyssal clays, according to Dr Murray, are pro- 

 duced by the decomposition in sea-water of floating pumice and of 

 ashes, ejected from volcanoes and ultimately falling into the sea ; 

 and there is evidence to show that they are the result of an exces- 

 sively slow process of accumulation. 



From a study of the deposits at present in process of formation in 

 the deep sea, far from land, it has been concluded by Murray and Re- 

 nard, as before said, that no similar or parallel deposits exist amongst 

 the varied marine sediments which compose so large a portion of the 

 present continents. On this view, the ordinary stratified rocks of 

 marine origin have all been formed in comparatively shallow water, 



