THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE V. VOL. I. 



No. v. — MAY, 1904. 



oiaioin^-A-Xj jLieTiGLES. 



I, — On some Samples of Rock from Borings at Sangregrande^ 

 Trinidad. — Part I. 



By E. J. Lechmere Guppy. 



(FOLDING PLATE VII.) 



I HAVE been favoured by P. N. H. Jones, Esq., engineer of tbe 

 Waterworks, with specimens from a boring at Sangregrande 

 undertaken by the Government of Trinidad with a view to ascertain 

 the extent and position of the Tertiary coal-seams of that district and 

 other facts. The specimens consist of a dark-coloured (blackish) 

 sand-rock, finely (but slightly irregularly) laminated, the lamination 

 being at an angle of about 50° from the horizontal. The samples 

 came from 150, 250, 400, 500, and 600 feet deep below the surface. 

 I examined each one separately, but as the differences between them, 

 whether as regards mineral constitution or organic contents, are only 

 slight, and in fact two portions from the same depth show often as 

 much difference as samples from different depths, I shall describe the 

 whole together. These specimens are from boring No. 3, Plate VII. 

 The rock is of a blackish or dark-grey colour, and is principally 

 composed of very fine sand with particles of mica. When washed 

 and passed through fine muslin only about xJo part I'emains. 

 Calcareous matter is under 10 per cent, in quantity, and exists 

 almost entirely as shells of Foraminifera and Mollusca. Crustacea 

 and echinoderm remains also occur, and there are numerous Polyzoa, 

 a few Ostracoda, and scales, teeth, and ear-bones of fishes. I could 

 not detect coccoliths or calcaroma, but the nature of the material is 

 unfavourable for their detection. The material of this rock is not 

 of any economic value, but it is interesting as throwing light upon 

 geological questions and upon the conditions which prevailed at the 

 time of its deposition. The neighbourhood of land is indicated by 

 the quantity of clastic material, but the fineness of the grains and 

 the tenuity of the laminae of deposition show that it was at some 

 distance off. Data exist for an approximate estimate of the position 

 of such land, and the elementary facts are indicated in my papers ir^ 



DECADE V. VOL. I. NO. V. 13 



