62 Mr Hatch, Note on a Remarkable Instance of 
Note on a Remarkable Instance of Complete Rock-disintegration 
by Weathering. By F. H. Harcn, Ph.D., Mem.Inst.C.E. 
[Read 25 November 1912.] 
THE material to be described occurs at Diamantina, in the 
province of Minas Geraes, Brazil, where it is being worked for 
diamonds. It consists of a conglomerate which, under the influence — 
of weathering, has been disintegrated to the condition of a loose 
sandy formation capable of being dug out with a shovel at the 
lowest depth yet attained in the open working. 
The Pebbles. The pebbles range in size from the smallest 
dimensions up to a maximum diameter of 3 inches. They have 
been worn perfectly smooth by water attrition and are rounded, 
generally to an ovoid shape. The materials of which they are 
composed are, in the order of relative abundance: 
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Quartzite. 
Vein-quartz. 
Steatite or soapstone. 
Tourmaline-quartz vein-stuff. 
The steatitic material, on account of its softness and consequent 
liability to pulverisation, rarely shows any smooth or rounded 
surfaces ; it crumbles between the fingers to an unctuous powder, 
resembling French chalk. No doubt this material has been 
derived originally from the decomposition of an ultra-basic igneous 
rock in which magnesian silicates (such as olivine) predominated. 
The other pebbles, viz. those of the quartzite, vein-quartz 
and tourmaline-quartz vein-stuff, are in a very friable condition, 
crushing to powder in the hand under the least pressure. This 
peculiar condition of pebbles of materials such as quartzite and 
vein-quartz, which in their normal condition are characterised by 
extreme hardness and unseparability, will be referred to later. 
The Sand. The sand consists of a mixture of colourless 
quartz and of the fine powder produced by the pulverisation of 
the soapstone fragments. By careful washing with water the 
slime formed by the soapstone can be removed, leaving a white 
quartz sand which under the microscope is found to consist of 
colourless angular grains, often presenting the characteristic 
pyramidal faces of quartz crystals. Rounded grains are rare, and 
