CONTAINING DIATOMACEOUS EXUVI^. 31 



shira, the waters of which flow into Loch Fine, well known 

 as a sea-loch, at its upper part. The sand occurs above a mile 

 from the mouth of the valley, lying under a considerable 

 depth of good alluvial soil. It is nearly black, with shining 

 particles of mica, and very dense. It consists chiefly of the 

 detritus of the surrounding mountains, formed of micaceous 

 schist, and contains therefore much quartz and mica. There 

 is also a considerable proportion of an iron ore, and of a dark 

 matter of vegetable origin, and apparently somewhat of a 

 peaty character. To the last-named ingredients the dark 

 colour of the sand is due. 



On placing a little of it under the microscope, I noticed one 

 or two Diatomaceous forms, such as a Navicula didyma, a 

 Cocconeis scutellum, and a Synedra radians. But the propor- 

 tion of these was so small that without some purification 

 nothing could be done. After various trials, I found the fol- 

 lowing plan to yield tolerably satisfactory results. 



The mass was first warmed, and when the violence of the 

 action had passed, boiled, with the most concentrated nitro- 

 muriatic acid. This not only dissolved the iron ore, but 

 completely removed the dark organic matter, and left a sand 

 of a pale-yellowish colour, in which the Diatomes were more 

 easily seen. 



The next step was to remove, by subsidence in water and 

 decantation, the greater part of the quartz and all but the 

 finest and lightest scales of mica, which, having much the 

 same density as the shells, could not be got rid of. Any 

 attempt to push the process farther caused a loss of shells. 

 The residue thus obtained was now found to be rich in Dia- 

 tomes ; and when mounted in Canada balsam, the mica became 

 so transparent as not materially to interfere with the exami- 

 nation of the shells. The entire residue did not exceed l-20th 

 of the original sand, and the Diatomes formed only from l-5th 

 to l-3rd of the residue, so that they could not have amounted 

 to much more than 1 or 2 per cent, of the mass. 



It will be seen from this, that the Glenshira deposit is of 

 an entirely different character from those earths in which 

 Diatomes have usually been found in the fossil state, such as 

 the Raasay or Mull deposits, which consist entirely of Diato- 

 maceous shells. On the other hand, it presents all the cha- 

 racters of a lacustrine or estuarial deposit or mud, such as the 

 Thames mud, or similar deposits now forming in estuaries or 

 lakes. Of course the predominant mineral ingredients are 

 such as are yielded by the adjacent rocks, and the Diatomes 

 have merely been deposited in small proportion along with 

 these. We shall see that there is a very remarkable analogy, 



