Vol. 51.] FOKMATIOtf OF OOLITE. 205 



structural portion and is either secreted or deposited material, 

 probably the former. Seeing then that the oolitic granules of the 

 Great Salt Lake and the Red Sea x have originated from the growth 

 of lime-secreting algae, it is possible that the fossil forms have a like 

 origin. 



I do not, however, wish to commit myself by saying that the 

 apparently tubulated forms of growth in oolitic granule? are allied 

 to the algae. This may be so, but, on the other hand, they may be 

 even lower in the scale of life. Girvanella is the first type of oolite- 

 forming organism discovered, and is simply a tubule. 



The exact process of formation seems to be this : — Minute fragments 

 of remains of calcareous organisms, such as corals, polyzoa, foramin- 

 ifera, crinoids, etc., collected on the floor of the sea. These became 

 nuclei to which the oolite-forming organisms attached themselves, 

 gradually building up a crust. Sometimes this growth was concentric, 

 sometimes at right angles to the nucleus, or the two combined. 

 When the growth was concentric other tubules frequently cropped 

 up in other directions and crossed the concentric tubules. At the 

 same time calcareous material was secreted, and the interstitial 

 spaces between the tubules were filled. 



The explanation of the formation of oolitic granules which I have 

 thus given is simple, and much more reasonable than to suppose that 

 they originated from chemical deposition of concentric strata around 

 a nucleus in the open sea, a condition of things contrary to what is 

 possible according to our present knowledge of the laws of chemical 

 combinations or precipitations. 



I cannot, moreover, refrain from pointing out the great interest 

 and beauty of the process of formation of oolitic granules by the 

 growth of such low forms of life as the calcareous algae — a work so 

 simple, yet so vast. 



EXPLANATION OF PLATE VII. 



Fig. 1. Oolitic granule from the Carboniferous Limestone near Buxton. The 

 nucleus consists of an aggregation of Girvaiiellee, and the encircling 

 crust is surrounded by tubular forms of growth of organic origin. 

 X 65 diam. 



2. Oolitic granule from the Forest Marble near Cirencester. X 65 



diam. 



3. Oolitic granule from the Forest Marble near Cirencester. This shows 



two distinct stages of growth. X 65 diam. 



4. a, & b. From the Forest Marble near Cirencester. The first of these 



is drawn from a positive photograph, and shows the object as seen 

 by transmitted light. The second (4 6) is drawn from a negative 

 photograph, and the granule appears as seen by reflected light. 

 The object of the latter illustration is to show the infilling or 

 secreted nature of the amorphous calcium carbonate, which appears 

 white, as against the actual structural portion of the granule, which 

 is dark. In the positive illustration (4 a) the colours are reversed. 

 X 65 diam. 



1 Op. supra cit. 

 Q.G.J.S. No. 202. 



