Geology. 461 



4. Certain Jurassic (Lias- Oolite) Strata of South Dorset; 

 and their Correlation ; Certain Jurassic (" Inferior Oolite") 

 Species of Ammonites and Brachiopoda j by S. S. Buckman. 

 Quart. Jour. Geol. Soc, London, lxvi, No. 261, February 1910, 

 pp. 52-108, pis. ix-xii. — The author describes here in detail 

 certain strata of the Jurassic of the Dorset coast and compares 

 them with similar strata inland. 



American geologists often doubt the ability of paleontologists 

 to make correlations of zones having thicknesses from 10 to 50 

 feet, but Buckman here makes correlations of zones but a few 

 inches in thickness ! He states : 



" The Inferior Oolite rocks are so remarkably prolific in species 

 of Mollusca and Molluscoids — there is so great a number of 

 species yet awaiting description — that there seems to be an idea 

 among those who have not studied the rocks in the field, that 

 they receive a kind of preferential treatment in this respect. 

 Another explanation may be suggested — that our judgment as to 

 the time occupied in the forming of the so-called ' Inferior 

 Oolite ' strata is warped owing to their tenuity — that they repre- 

 sent a time during which destruction of strata was particularly 

 active, therefore the remaining deposits are only the fragmentary 

 representatives of a whole. 



" This is particularly the case in Dorset. A schoolboy once 

 defined a net as a series of holes strung together, and the Dorset 

 Inferior Oolite might be defined as a series of gaps united by 

 thin bands of deposit. And one reason for the prolificness of 

 the deposits is that the amount of deposit can be no indication of 

 the amount of time, as shown by the changes in successive 

 faunas ; and also that the deposits are so local — the deposits of 

 one place correspond to the gaps of another. Therefore many 

 localities have to be placed together to produce the full tale of 

 the Inferior Oolite. The very local distribution of Inferior 

 Oolite species often means that strata of particular dates have 

 only been preserved in a few favoured localities. 



" The beds of the l Inferior Oolite,' in a restricted sense, have 

 now been divided as deposits of about twenty-two successive 

 dates or hemerse. The total for the whole of the Jurassic would 

 not be more than about eighty-five, or perhaps, on an extended 

 scale, a hundred hemerse. Therefore, according to this reckon- 

 ing, the few feet of Inferior Oolite represent from about a 

 quarter to a fifth of the total time occupied in the deposition of 

 the entire Jurassic System. 



" One can hardly view the few feet of Inferior Oolite lime- 

 stone at Burton Bradstock, about 15 to 20 feet say, and imagine 

 that it represents an interval of time equal to a quarter or a fifth 

 of the whole Jurassic Period — a time during which thousands of 

 feet of strata were laid down. But this is because we do not 

 allow sufficiently for the gaps. 



"If anything like this supposition be correct, then the Inferior 

 Oolite prolificness is understandable ; it should represent in 



